The present invention relates generally to the field of fetal monitoring and more specifically to the photoplethysmographic measurement of oxygen saturation and heart rate from a fetus during labor and delivery.
Pulse oximeters are commonly used in adult, pediatric, and neonatal care to provide a measurement of arterial oxygen saturation. A pulse oximetry system typically consists of a sensor which is applied to the patient, a monitor on which the measurements of arterial oxygen saturation are displayed, and a cable which connects the sensor to the monitor. The sensor typically contains light emitting diodes whose output light is incident on the surface of the tissue-under-test and a photodetector that measures the intensity of the light exiting the tissue-under-test at the sensor site.
The sensor does not necessarily have to contain the emitters for the delivery of light to the tissue-under-test. Light can be transmitted to the sensor from the emitters via optical fibers. The use of one or more optical fibers allows the emitters to be located some distance from the sensor. Light from the emitters can be coupled into the fibers, and the distal ends of the fibers, located in the sensor, become the light emitter. It is also possible to use optical fibers as the photodetector for receiving light from the tissue-under-test, but this tends to result in a very small signal level compared to placing a photodetector, such as a photodiode, directly on the surface of the skin.
The sensor cable must contain the electrical and/or optical conductors for powering the light emitters and for conducting the electrical or optical signals from the photodetector back to the monitor for analysis and conversion to the measured parameters. If the LEDs and photodiode reside in the sensor, the conductors will be electrical wires. If the emitters and/or photodiode (or other similar device, such as a phototransistor, for conversion of the detected light to an electrical signal) reside somewhere other than directly in the sensor, the conductors will be optical fibers or a mixture of electrical and optical conductors.
While arterial oxygen saturation is the most commonly measured blood analyte, it is only one of several blood analytes that are, or can be, measured by photoplethysmography, the monitoring technology used in pulse oximetry. Other blood analytes that can be measured include carboxyhemoglobin and methemoglobin. Hemodynamic parameters measured by photoplethysmography include heart rate and perfusion index, an indicator of the blood perfusion of the tissue-under-test at the sensor site. The tissue-under-test is the tissue that the light emitted from the sensor passes through before being detected by the photodetector.
The use of pulse oximetry has recently been expanded to include its use on a fetus during labor and delivery. U.S. Pat. No. 5,228,440 reveals a fetal pulse oximetry sensor which is intended to be positioned on the fetal cheek or side of the fetal head. This sensor does not adhere to the fetus and is therefore sensitive to changing position with respect to the fetus as a result of contractions during labor and progression of the fetus through the birth canal. This movement of the sensor with respect to the tissue-under-test often results in a loss of signal, thereby necessitating periodic repositioning of the sensor. In addition, the application of the light emitter and the photodetector to the same surface of the tissue-under-test, versus placement across the tissue-under-test such as when the light emitter and the detector in the sensor are placed on opposite sides of a finger, allows the possibility of the emitted light being shunted directly from the light emitter to the photodetector without passing through the tissue-under-test. This can cause the fetal pulse oximetry readings to be erroneous.
Alternate methodologies for fetal pulse oximetry have been considered that make use of a modified version of the fetal spiral electrode, a device designed and manufactured for the measurement of the fetal electrocardiogram (ECG). This spiral ECG electrode is disclosed in FIGS. 8, 9, and 10 of U.S. Pat. No. 3,827,428. The spiral, or more accurately “helical”, ECG electrode in combination with fetal pulse oximetry has been presented in a number of different potential configurations.
In U.S. Pat. No. 5,154,175 the helical electrode is used to hold the light emitter and photodetector elements flush against the fetal scalp, the tissue-under-test. While this sensor remains fixed with relation to the fetus, it still has the problem that both the light emitter and the photodetector lay on the same surface of the tissue-under-test. This allows the possibility of errors in readings caused by light being shunted directly from the emitter to the detector without passing into or through the tissue-under-test.
Two patent documents, U.S. Pat. No. 5,361,757 and U.S. Patent Application Publication No. 2005/0283059 A1, disclose a potential solution to this problem. In the first of these two publications, the emitters are light emitting diodes (LEDs) which are positioned at a window in the helical needle. When the sensor is in position on the fetal scalp, the light is emitted subcutaneously into the tissue-under-test and detected when it emerges from the tissue at a detector in the base of the sensor on the surface of the fetal scalp. U.S. Publication No. 2005/0283059 A1 reveals a slightly different arrangement in which both the LEDs and the photodetector are positioned in the helical needle. In this arrangement the light is transmitted subcutaneously from the light emitters directly across to the photodetector, given that both elements are located under the surface of the skin once the sensor is in place on the fetus.
The problem common to both of these solutions is the extremely short pathlength that the light traverses in the tissue-under-test before reaching the photodetector. Photoplethysmography requires that the light passing through the tissue-under-test be modulated by the pulsating blood flow thereby creating a pulsatile light signal at the photodiode. With the extremely short physical pathlength of these previous sensor configurations, the light passes through very little pulsatile tissue which results in a very small pulsatile signal. The end result is a poor signal-to-noise ratio and inaccurate photoplethysmographic readings. It is necessary to have a sufficiently long pathlength for the light to traverse the tissue-under-test to create a large pulsatile component in the received light signal for calculation of the measurement parameters.
Designing a fetal sensor that provides a sufficiently large pathlength through the tissue-under-test, typically the fetal scalp, creates a new problem because it necessitates a large physical size for the sensor. The fetal sensor is intended for use during labor and delivery and preferably such a sensor would be placed on the fetus as early in the progression of labor as possible. If the sensor is large, it requires greater dilation of the cervix before it can be placed on the fetus. The greater the dilation required for sensor placement, the longer the clinician must wait during labor before the sensor can be placed and the less valuable a clinical tool it becomes.
The solution to the problem of how to create a fetal sensor with a sufficiently long tissue pathlength, while still minimizing the size of the sensor during insertion to allow early placement, is the subject of this invention.